Fix conversion to JSON of empty YAML arrays (#683)

We were not initializing arrays when converting from YAML to JSON, so
the ones that don't have elements were kept as null. This lead to
unexpected error messages about unexpected null values.

code/go/internal/yamlschema/loader.go

@@ -113,7 +113,7 @@ func expandItemKey(c interface{}) interface{} {
 
 	// c is an array
 	if cArr, isArray := c.([]interface{}); isArray {
-		var arr []interface{}
+		arr := []interface{}{}
 		for _, ca := range cArr {
 			arr = append(arr, expandItemKey(ca))
 		}

code/go/pkg/validator/validator_test.go

@@ -172,7 +172,6 @@ func TestValidateFile(t *testing.T) {
 			"data_stream/foo_stream/manifest.yml",
 			[]string{
 				"field streams.0.vars.1: options is required",
-				"field streams.0.vars.2.options: Invalid type. Expected: array, given: null",
 				"field streams.0.vars.3: Must not be present",
 			},
 		},

spec/changelog.yml

@@ -12,6 +12,9 @@
   - description: Prepare for next version
     type: enhancement
     link: https://github.com/elastic/package-spec/pull/681
+  - description: Fix validation of empty arrays in YAML files
+    type: bugfix
+    link: https://github.com/elastic/package-spec/issues/683
 - version: 3.0.3
   changes:
   - description: Do not require an object_type on objects with `enabled` set to `false`

test/packages/good_v2/data_stream/foo/fields/some_fields.yml

@@ -91,3 +91,6 @@
   expected_values:
     - artifact
     - directory
+- name: empty_expected_values
+  type: keyword
+  expected_values: []

test/packages/good_v3/data_stream/foo/fields/some_fields.yml

@@ -86,3 +86,6 @@
   metrics:
     - min
     - max
+- name: empty_expected_values
+  type: keyword
+  expected_values: []